1. Field of the Invention
The present invention relates to a supporting structure of bearings for a rotor which is supported at more than two points, and more particularly in a supporting structure of bearings which is used to a mechanical supercharger such as a roots-typed mechanical supercharger.
2. Description of the Prior Art
A conventional supporting structure of bearings of this kind is disclosed in, for example, Japanese patent application laid-open publication No. 2 (1990)-91492. FIG. 4 shows a roots-typed mechanical supercharger which is disclosed in this prior publication. Referring to FIG. 4, rotors 101, 102 are fixed on shafts 103, 104, respectively the shafts 103, 104 are rotatably supported on a housing 115 and on a bearing housing 116 at both sides of the rotors 101, 102 through radial ball bearings 106, 107, 108 and 109, respectively. At one end of the shaft 103, a driving pulley 110 is fixed thereto. The other end of the shaft 103 is projected into a gear chamber 112 which is formed by the securing a cover 111 to the bearing housing 116. A timing gear 113 is fixed to the other end of the shaft 103 in the bearing housing 116. The other end of the shaft 104 is projected into the gear chamber 112 and a timing gear 114 which is engaged with the timing gear 113 is fixed to the other end of the shaft 104. The timing gears 113, 114 permit the rotors 101, 102 to rotate simultaneously while maintaining a predetermined clearance therebetween without contacting with each other. Inner races of the radial ball bearings 106, 107, 108 and 109 are fitted on the shafts 103, 104 so as not to be able to move relative to the shafts 103, 104, respectively. Outer races of the radial bearings 107, 109 are fitted into receiving holes of the bearing housing 116 so as not to be able to move relative to the bearing housing 116. The outer races of the radial bearings 106 and 108 are fitted into receiving holes of the housing 115 so as to be able to move in the axial direction. Cone springs 117, 118 are disposed between bottom surfaces of the receiving holes of the housing 115 and the radial ball bearings 106, 108 so as to urge the outer races of the radial ball bearings 106, 108 toward the outside of the receiving holes of the housing 115.
FIG. 5 is an enlarged view of a portion A shown in FIG. 4 and FIG. 6 shows the fundamentals of the supporting structure of the bearings shown in FIG. 4. Referring to FIG. 5 and FIG. 6, the outer race 106a of the ball bearing 106 can freely slide on the inner circumferential surface of the housing 115 in the axial direction and is urged toward the outside of the receiving holes of the housing 115 (leftward in FIG. 5) by the cone spring 117. The inner race 106b of the ball bearing 106 and the inner race 107b of the ball bearing 107 are fixed to the shaft 103 and the outer race 107a of the ball bearing 107 is fixed to the bearing housing 116. Since the cone spring 117 urges the outer race 106a leftward in FIG. 6, the shaft 103 is axially moved through the ball 106c and the inner race 106b until the axial gap and the radial gap disappear in the ball bearings 106, 107. Thereby, it is possible to reduce the clearance between the rotors 101, 102 and the clearance between the rotors 101, 102 and the housing 115. Therefore, it is able to decrease a leak of the fluid in the supercharger and the volume efficiency (the practical amount of discharged flow/the theoretical amount of discharged flow) can be improved.
Generally, when bearing steel used as a material of the outer and inner races of the bearing differs from the material of the shaft and the housing, the fit between the outer race and the housing and the fit between the inner race and the shaft become loose at high temperatures as a result of differences in the coefficients of thermal expansion between the different materials.
In particular, in case of the supercharger, a coefficient of thermal expansion of bearing steel normally used as a material of the outer race of the bearing differs from that of aluminum alloy normally used as a material of the housing. Thereby, when the temperature of the supercharger itself becomes high in operation in an atmosphere at a high temperature or in operation under high-load condition, the fit between the outer race of the bearing and the inner circumferential wall of the housing becomes loose. Further, in general, grease is sealed between the outer and the inner races of the bearing and the torque of the inner race is transmitted to the outer race through the grease (=by the agitation resistance of the grease) under the condition in which the inner race of the bearing is rotated with the shaft in a body.
Accordingly, in the above prior supporting structure, when the fit between the outer race of the bearing and the inner circumferential wall of the housing becomes loose in operation in an atmosphere at a high temperature or in operation under high-load conditions, the outer race urged by the cone spring is apt to be rotated by the agitation resistance of the grease and namely what is called, the creeping rotation of the outer race is easily generated. When the creeping rotation is generated remarkably, the fitting portion between the outer race of the ball bearing and the inner circumferential wall of the housing wears down. Therefore, in case that the above supporting structure is applied to the supercharger, for example, there exist the danger that interference between the rotors or between the rotor and the housing will occur.